Motion Control Goes to Sea
| By: Kristin Lewotsky, Contributing Editor
From the docks to the deck, motion control provides big benefits for marine applications.
In the maritime world, motion control technology is used in everything from loading cargo and optimizing propulsion systems onboard ship to running cranes and gantries on the wharfs. The problem is that the marine environment makes operations in even the dirtiest factory look like a piece of cake. From shock and vibration to extreme temperatures to corrosive salt spray, operations at or near the sea take place in some of the harshest conditions around.
Beyond that, the marine environment presents some unique challenges. In addition to heat and the salt air, electromechanical components that power portside cranes, for example, need to survive wind-driven vibration. Onboard ship, cranes and internal systems suffer additional stresses from engine vibration, not to mention the buffeting of the waves. "On a ship, everything's trying to change from a rectangle into a parallelogram," says Brian Winter, Encoder Product Manager at Avtron Industrial Automation Inc. “That constant bucking and heaving is why you have to have special elements in your drives to maintain electrical contact under repeated stress and release cycles.” Meanwhile, shipboard electrical systems tend to provide poor-quality power with transient effects and surges. Add high humidity and a 24/7 duty cycle and you’ve got an environment designed to destroy electronics.
One of the most important characteristics for marine systems relates not to performance but to product lifespan. Port infrastructure and vessels are typically designed for 20- and 30-year life cycles. Designers don't want products that are updated year-to-year, they want to specify a product that their end users will be able to buy for two or three decades. Designs need to be forward and reverse compatible.
Motion at Sea
In the Coast Guard, motion control helps optimize propulsion systems, increasing performance and saving fuel. In 2009, the U.S. Coast Guard began to refurbish the 13 ships in its medium-endurance cutter (WMEC) class. Built between 1983 and 1991, the 270-ft., twin-screw vessels were nearing the midpoint of their lifetime. The plan was to bring each of the cutters into drydock for a complete refit that would leverage the latest technology to optimize performance while extending their lifetime for another 10 to 15 years.
The propellers that drive ships can be divided into two classes: fixed pitch and variable pitch. As the name suggests, a fixed-pitch propeller (FPP) features static blades. Blade pitch is key to the performance of the propulsion system - if the blades are positioned for maximum speed, performance at low speed is compromised. If they are positioned for maximum power at low speed, the vessel may never reach peak velocity. As a result, a ship fitted with FPPs suffers significant limitations in both performance and efficiency.
Variable-pitch propellers (VPPs) provide a solution. Because the position of the propellers can change in real time, VPPs optimize performance and efficiency at all speeds. Matching blade pitch to engine speed can provide efficiency increases of 3% to 4%, which can generate significant savings in fuel. That's important for the Coast Guard, which, like everyone else, is trying to do more with less.
The propulsion systems of the WMECs consist of two 3650 hp v18 ALCO diesels, each fitted with a five-blade, 9-ft.-diameter VPP. Each blade features a hydraulically controlled pitch setter to adjust position and balance performance between the two screws. In the original design, an oil distribution (OD) box controlled by a custom-designed stepper motor ensured accuracy and repeatability of the hydraulic positioning. When it came time to do the retrofit, the Coast Guard elected to change from stepper motors to servo motors. "The biggest issues were serviceability, logistics, and obsolescence," says Joe Moffa, Director of Marine Programs of Rockwell Automation Inc. (Philadelphia, Pennsylvania). "Before, they were using a custom-built stepper-motor configuration. Now they’re using commercial off-the-shelf (COTS) components so they’re not only gaining accuracy, they’re gaining life cycle support.”
Each OD features a pilot valve which rotates roughly 95°, and a feedback shaft with a similar range of motion. The design team chose permanent-magnet servo motors with planetary gearboxes for a reduction ratio that provides rotational speed on the order of 1.9°/s. The pitch setter resides in the engine room where space is at a premium and temperatures can soar to 130° F. Although the servo motors need to be mounted directly on the OD box, the drives are located remotely, where the space and temperatures are more manageable. For shaft position feedback, engineers went with a dual-independent sensor unit. This provides the system with both precise resolver-based position display and independent feedback for closed-loop pitch position control. Having an independent feedback device also limits the number of failure modes to minimize the chance of fault.
One key requirement of the new servo-based system was for it to continuously monitor the motor current and automatically limit torque for cases in which the pilot valve did not respond as designed or expected. “This allowed us to create a system design which is inherently resistant to inadvertent mechanical damage when the hydraulic system becomes jammed or stalled for any reason.” says Chris Jones, Senior Application Engineer at Rockwell.
Perhaps the single biggest challenge in marine applications is the low fault tolerance. In manufacturing, downtime just costs money. When it comes to shipboard operations, downtime can be catastrophic. “There was a tremendous amount of effort made to ensure that the propeller pitch system was as reliable as possible because you don’t want to be in the middle of rough seas or a mission and lose control," says retired Navy Captain John Langan, business lead for Navy and Coast Guard Programs at Rockwell. That's the critical nature of the application. It's the machine safety of the ship but it's the life safety of the crew aboard.”
As a result, the design leverages smart components in a networked system to provide diagnostic information for predictive maintenance. The pitch setters are wired into the machine control monitoring system (MCMS) that runs the engines, streamlining operations. “What this really does is allow the crew to operate their plant from a central control station, so it minimizes the various watch standers who have to run around and check the various individual components," says Langan. "They can have them in a centralized space where they can control and monitor much more accurately and efficiently."
Ports of Call
A ship that's not in motion is losing money. As a result, ports around the globe vie with one another for speed of loading and unloading, which means they can't afford a crane failure. The problem is that the motion components in those units need to survive conditions that are, if possible, even worse than those found in a shipboard engine room - not just humidity, vibration, and high duty cycle but salt spray, washdown, and wide, rapid temperature swings. “We're always focused on withstanding this oppressive environment - the salt air, the salt water, the temperature changes, the less-than-perfect electricity,” says Winter. “These cranes have to run 24 hours a day, seven days a week, and if they don't there's a fierce penalty, because the people that you have the cargo contracts with may go elsewhere.”
Avtron starts by using copper for their busbars, protecting it with special plating. They encapsulate their circuit boards in blocks of plastic. To lessen the environmental effects on drives, they focus on centralized architectures, enclosing the drives in cabinets.
When it comes to encoders, however, there’s no choice - the devices have to be out by the motors, and if the motors power deck cranes, the feedback devices have to endure punishing treatment. "It's a matter of surviving the endless hot and cold cycles," says Winter. "An encoder might be on a motor hoisting something and heat up to 70° C, then seawater hits it at 0° C. The hardware has to withstand fierce and continual temperature cycling.” Using magnetic rather than optical sensors ensures that feedback remains robust even in the face of moisture and contamination. Eliminating bearings removes another point of failure. Beyond that, encapsulation is the single most important means of protecting components and ensuring lifetime. Coatings are not enough - the components need to be protected by a material that’s impervious to humidity and contamination.
Intelligent components that provide diagnostic information are essential, Winter says. “What we really see is people wanting complete and integrated systems so they can look from the top down and see that everything is working, and if it isn't they can quickly break it down to the faulty item or system and attack it."
Speaking of faults, it's a well-known fact that cabling presents the single most common point of failure. That's a particular problem for cranes, which feature festoons of cabling that undergo constant flexing and extension. Even worse, the long runs can degrade signal quality. “If you don't have a lot of power behind it, a perfectly pretty square wave on the front end looks like a tiny little wiggle on the back end,” says Winter. “You can't tell whether it's an encoder signal or noise.” The solution is to increase the output signal from the encoder. “Even if your wire has a cracked shield or a bent conductor or something, you can still force enough current through so the drive can still see the signal and get correct sense of the position."
The Marine environment presents a unique set of challenges for motion control, but in return, motion control brings a unique set of benefits. With careful design, engineers and systems designers can successfully build robust, repeatable, high-performance systems that leverage the power and flexibility of electromechanical shafting.